Homopolar machines are operated by direct current (DC) and are simple in design principle. They have been under consideration and development for some years for use in ship propulsion applications because of their high efficiency, compact size, low weight, and reduced acoustic signature relative to all other motors, see U.S. Pat. No. 3,657,580 (1972).
A homopolar machine includes four major components: armature, stator, field coils, and flux return. The armature is connected to the machine's shaft and is often referred to as the rotor. The armature typically includes a series of concentric, copper cylinders and is free to rotate in a direction at right angles to the magnetic field lines produced by the field coils. When a voltage is applied across the armature in the direction of the shaft, electric current flows parallel to the shaft. The current and magnetic field interaction (I×B) results in torque generation and rotation, thus producing a motor. In contrast, if the armature is driven externally as a generator application, the interaction of the armature rotating at right angles to the magnetic field lines generates a voltage and electric current.
In both the motor and generator scenarios, current flows along the armature and to the stationary stator via sliding electrical contacts referred to herein as current collectors or brushes, which may take various forms and be made of various materials. Such materials include but are not limited to flexible fibrous copper and flexible copper strips; flexible copper fibers are perhaps most commonly used in DC motors.
The field coils are typically circumferentially continuous in geometry and aligned on the same central axis with respect to each other. A homopolar machine always cuts (or crosses as it rotates) magnetic flux lines of a magnetic field in the same direction due to the interacting armature and shaft iron being aligned on the same axis. This means that any point on the rotor always sees the same magnetic field as it rotates, and no differences in magnetic flux or multiple magnetic poles are encountered by conductive elements of the armature as it rotates. Hence the nomenclature “homopolar machine”.
The flux return typically includes a highly magnetically permeable material, such as iron or steel. The flux return is designed primarily to limit the undesirable stray magnetic field that radiates from the machine; therefore, it typically takes the form of a structural housing that surrounds the motor. In addition, the flux return may also be designed to help direct the magnetic field lines produced by the field coils into the armature interaction region to improve the machine's flux utilization.
Although conventional rotating machines are in wide use, brush wear has been a point of continuing concern, and one objection to homopolar machines is that they tend to often have lower reliability in comparison to standard DC motors. Because homopolar machines use current collectors, i.e. brushes, to transfer current between each rotating armature turn and each stationary stator turn, the value of homopolar machines is heavily dependent upon the reliability of such current collectors. Current collectors need to maintain uniform contact pressure with the armature, usually along slip rings, and performance is often measured in terms of current collector wear and current-carrying capability. Maintaining an ideal contact pressure and minimizing wear in a homopolar machine is the subject of U.S. Pat. No. 6,873,078, where homopolar machines are disclosed having where mechanisms that help to maintain ideal contact pressure and where it is taught that, in a homopolar motor, it has been found that the positive polarity (anode) brushes wear at a rate ten times higher than the negative (cathode) brushes. It is thus stressed that lifetime can be improved by polarizing all brushes negative.
Furthermore, the magnetic field that is inherent in homopolar and other motor concepts causes slip ring voltage gradients that induce high circulating currents in the brush. As a consequence, during motor operation, the circulating and transfer currents interact with the magnetic field resulting in substantial electromagnetic forces on the brush. Unless properly restrained, these forces can cause the brush to distort to the point where it is no longer functional.
Thus, the search has continued for better solutions to such problems.